MOSCOW (AP) — An outbreak of fighting in Ukraine’s rebel-held east has killed four troops and left another seven wounded, officials said Thursday.

The Ukrainian Defense Ministry said the losses were the biggest in months and followed fighting that lasted five hours.

The ministry said the fighting erupted when the rebels began to shell government troops with mortars, trying to break through the front line in the east of the Luhansk region.

The rebels in Luhansk, however, accused government troops of attacking them first. They said they fired back when the Ukrainian troops launched an offensive in a bid to seize some ground near the village of Zhelobok.

The separatist conflict in eastern Ukraine has killed more than 10,000 since it began in April 2014. A 2015 peace agreement has helped reduce hostilities, but clashes have continued. The warring parties blamed each other for the failure to observe the truce.

Ukrainian President Petro Poroshenko on Thursday apologized to the country for his 2014 promise to quickly end the conflict in the east.

“People perceived it as an opportunity to end the war quickly,” Poroshenko said. “I am sorry to have created inflated expectations. I sincerely apologize for giving you hope that has not come true.”

Ukrainian President Petro Poroshenko, center, poses for photo with soldiers during a rehearsal for the Independence Day military parade in Kiev, Ukraine, Wednesday, Aug. 22, 2018. Ukraine will mark the 27th anniversary of the Independence Day on Aug. 24. (Mikhail Palinchak/Presidential Press Service Pool Photo via AP)